The present invention relates to a system for controlling the air-fuel ratio of an air-fuel of mixture for an automotive engine, and more particularly to a system for controlling the air-fuel ratio in accordance with a feedback signal from an O.sub.2 -sensor for detecting the oxygen concentration of exhaust gases from the engine.
Generally, the engine is provided with a carbon canister for absorbing the fuel vapor in a fuel tank during the time when the engine is not running, and for purging the fuel vapor from the canister to an intake manifold at predetermined conditions of the engine operation. When the fuel in the canister is purged, the fuel vapor is added to the air-fuel mixture induced in the cylinders of the engine, rendering the mixture rich.
Referring to FIG. 6, a conventional canister purge system comprises a carbon canister 12 having a purge valve 15 and a solenoid-operated control valve 13 provided in a line 14 which communicates the purge valve 15 with an intake passage 3. The canister 12 is communicated with the intake passage 3 downstream and upstream of a throttle valve 3a through the purge valve 15 and a purge line 16. The control valve 13 is opened when its solenoid is energized by a signal current from an electronic control unit 20. Accordingly, vacuum communicating with the purge valve 15 through the line 14 opens the valve so that the fuel vapor trapped in the canister 12 is purged into the intake passage 3 through the purge line 16. In order to stabilize the air-fuel ratio when the engine is idling, an idle signal is applied to the control unit 20 to de-energize the solenoid of the control valve 13 to close the purge valve 15, thereby stopping the purging of the fuel vapor.
In another type of a canister purge system shown in FIG. 7, the canister 12 is communicated with a purge port 3b formed in a throttle body immediately above the throttle valve 3a through the purge line 16. When the engine is idling, the throttle valve 3a is closed, so that vacuum does not communicate with the canister 12, thereby cutting off the purging of the fuel vapor.
When the vehicle is driven where the atmospheric temperature is high, or at high altitude, a large amount of fuel vapor is generated so that when the canister is purged, the air-fuel ratio becomes excessively rich. Accordingly, the air-fuel ratio control system operates to dilute the rich mixture in accordance with the feedback signal of the O.sub.2 -sensor. Namely, a feedback correcting coefficient is set to a minimum value (for example, 0.75), so that the air-fuel ratio is maintained at the stoichiometric air-fuel ratio. Under such a condition, when the throttle valve is closed at idling, the purging is cut off. Accordingly, the air-fuel mixture induced in the cylinders immediately becomes lean. The feedback control system operates to enrich the mixture by increasing the correcting coefficient in response to the output of the O.sub.2 -sensor. However, as shown in FIGS. 8b and 8c, because of delay of the feedback control operation, the air-fuel mixture stays lean for some time, which will cause the engine to malfunction.
Japanese Patent Applications Laid Open 58-35256, 59-188063 and 60-175757 disclose systems for preventing the air-fuel mixture from becoming too rich as a result of the purging of the canister. Japanese Patent Applications Laid Open 60-8458 and 61-1857 disclose systems wherein deviation of the air-fuel ratio during the purging of fuel vapor is prevented in dependency on the feedback signal of the O.sub.2 -sensor. However, none of these disclosures proposes a control system of the air-fuel ratio where the delay of the feedback control is compensated when the purging of the fuel vapor is stopped.